1. Field of the Invention
In one aspect this invention relates to engine accessory drives useful with internal combustion prime movers. In a further aspect this invention relates to an engine accessory drive that is adapted to operate a driven accessory device at an efficient speed at low or tactical idle engine rpm but prevent over driving the device when the engine is operating normally.
2. Prior Art
In general, the internal combustion engine used in land vehicles drives one or more accessory devices through a combination of pulleys, belts, gears and drive shafts generally using power taken from a driving element i.e., the engine crankshaft, camshaft or flywheel gear. Examples of common accessories powered using the driving element in a military vehicle include: alternators, air conditioning, hydraulic pumps, cooling fans, power steering and water pumps. In a normal civilian vehicle, each accessory device will be equipped with a driven pulley which is sized so the accessory is driven at an acceptable output at low engine speed but which will not overdrive the accessory at normal engine operating rpm. By properly sizing the driven pulley on each accessory and some compromising of accessory design parameters, an acceptable level of accessory performance over the range of normal engine rpm conditions can be achieved. This requirement to compromise creates certain problems. For example, alternators are generally designed so they achieve their rated output when the engine rpm is at cruising speed or faster. Thus at normal idle, the alternator output will be well below its peak capacity. By way of illustration, an alternator having a rated capacity of 100 Amps will generally be designed and have a pulley so it is driven with a rotor speed of about twice the engine rpm. At an engine rpm of about 650 the rotor will be turning at about 1300 rpm and using the same 2:1 drive ratio the rotor speed would be about 2600 rpm at the military tactical idle. Referring to published power curves for 100 amp alternators, this will generate about 40 amps at an engine idle rpm of 650, 80 amps at tactical idle and 100 amps at normal engine cruising rpm of 2200. It is apparent that a 40-amp output leaves little current available at idle speed to operate systems or recharge batteries. Since starting an internal combustion engine is a considerable drain on the battery, the idle speed output does not generate sufficient current to recharge the battery in a reasonable length of time. For tactical military vehicles this problem is compounded by a duty cycle characterized by extensive periods of time with the engine idling while running accessories such as radar, radio and other electrical or accessories. Thus, it is desirable to have a mechanism where the alternator can power accessories at or near their maximum operating level even when the engine is idling for an extended period of time without the need to draw substantial power from the batteries.
One example of a commercially wide spread accessory drive used to match accessory power with system requirements is the viscous fan clutch used in automobiles. This device was developed to ensure sufficient cooling air was furnished to the radiator for proper engine cooling allowing higher fan speed when the coolant is hot or the airflow through the radiator is minimal. However, the clutch will reduce or eliminate the fan power requirements when enough cooling air was present without the fan""s assist.
Another adjusting drive mechanism having more than one driving speed used with vehicle accessories is a variable speed pulley. This configuration where a v-belt drive rides in an adjustable pulley sheave, the sheave diameter and therefore the drive speed can be adjusted to cause a difference in accessory speed based on the required output and prime mover rpm. U.S. Pat. No. 4,969,857 represents one example of a variable drive pulley.
Military requirements are to a large extent unique, and the normal tradeoffs in drive speed and performance acceptable in civilian operation may not be acceptable in the military environment; particularly, since failure of military vehicles can result in death of service members. One of the critical operating requirements experienced by military vehicles and shared by civilian emergency vehicles is a need to stand with its engine idling for extended periods. Military vehicles generally are made with the ability to operate at what is referred to as tactical idle, which is an idle speed considerably above the normal commercial engine idle of 650 rpm at the crankshaft. Tactical idle may be in the range of 1000 to 1200 rpm at the crankshaft when the vehicle is stationary. While tactical idle is higher than the normal resting idle, it is not high enough for the alternator to reach the output level necessary to rapidly and fully recharge the batteries or provide sufficient power for other applications if driven at the normal 2:1 ratio discussed before. However, it does not solve the problem to merely change the drive ratios of the accessories, because if the alternator""s rotor were driven at a higher rpm at idle, it would be seriously over driven and destroyed at highway cruising speeds.
What is needed, from a military perspective, is a simple reliable drive system for accessories that delivers the necessary accessory output i.e., current, hydraulic pressure, etc to perform mission functions over a range of engine speeds from standard to tactical idle rpm. But, the drive system should return the accessories to normal operating conditions when the vehicle is moving and operating in a normal mode. Since normal operations will include starting and stopping, it is desirable that the device which controls the speed of the accessory not reset every time the engine comes to idle rpm temporarily. However, an increased accessory speed is necessary when the vehicle is in an idle mode more than the time normally spent waiting to resume normal driving conditions and the device will operate accordingly. A further consideration is the drive must have a default provision that the drive goes to the low setting used for normal operation since the vehicle must have the capacity to move and xe2x80x9climp homexe2x80x9d in the event of drive malfunction .
Briefly the present invention is designed to address the problems noted above. The device of this invention is generically a variable speed accessory driving combination with a relatively higher driving speed at lower engine rpm and a normal driving speed at normal engine cruising speed useful with a vehicle having a prime mover. The prime mover can be an internal combustion engine or similar source of power. The device has a power input driven by the engine crankshaft Thus input of this invention will have a speed proportional to the engine rpm.
A transmission is connected to and driven by the power input, the transmission having more than one drive ratio. The relatively high drive position will be engaged at idle speed to provide a correspondingly high accessory output relative to engine rpm. The relatively lower driving positions will function as a direct drive or in the case of a drive malfunction the default position and provide the normal auxiliary output common to land vehicle operation when the vehicle is not in an idle mode. The transmission""s drive position is controlled by an activator, which will move the transmission to the desired drive position based upon certain preset operating parameters.
The device has an associated sensing means that measures one or more chosen engine functions, generally engine rpm, to determine in which drive position the transmission should operate. The sensing means will generate a signal in response to the monitored function, which in turn will cause the activator to move the transmission to the desired driving position. For example, drive position could depend on the idle speed of the engine combined with the time the engine has been operating at the idle speed.